Basil Zempilas replaced Libby Mettam on Tuesday as the head of the party and promised a new era for the Liberals.
"This is a new-look Liberal Party of Western Australia," he told reporters on the steps of the state parliament in Perth.
"A reset is under way and the road to 2029 begins today."
The Perth media personality and former Lord Mayor of the city said he would lead "collaboratively, harmoniously and decisively in holding this government to account".
"Every decision we make, every item that we raise with the Cook Labor government of Western Australia will be raised with the intention of making that government better," he said.
Mr Zempilas, who narrowly won the seat of Churchlands at the election on March 8, said the party "did not resonate the way we would have hoped at the previous election".
"The way to step forward, the way for the reset of the Liberal Party in Western Australia to really begin, to gather momentum ... is to be out in the community and listening," he said.
Latest counting in the state poll shows Labor won 46 seats of 59 seats, the Liberals seven and the Nationals six, after the Liberals claimed the final lower house seat of Kalamunda on Monday night.
"By winning Kalamunda, we have seven members of the lower house and that is more than three times what we had at the corresponding time of the (last) parliament," said Mr Zempilas, a former footballer, television and radio presenter and Seven Network sports commentator.
"That in itself is a pointer towards the growth of the Liberal party."
Mr Zempilas is also set to become the state's Opposition leader after the WA Liberals' win in Kalamunda.
The Nationals were the opposition after Labor's 53-seat landslide 2021 election victory left the Liberals with just two lower house seats to the Nationals' four.
Ms Mettam has become deputy leader after she announced she would not seek re-election for the leadership.
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